Base station emulator

ABSTRACT

A wireless digital telephone system containing at least one emulated base station plus one or more subscriber stations, the emulated base station comprising a station similar to the subscriber station but having the capability of initiating a synchronization process whereby it is enabled to assign time slots to the subscriber station within the frame pattern of an amplitude signal by means of monitoring for positive edges in the signal.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/356,845, filed on Jul. 19, 1999, which is a continuation ofapplication Ser. No. 08/796,973, filed on Feb. 7, 1997, which issued onJul. 27, 1999 as U.S. Pat. No. 5,930,297, which is a continuation ofapplication Ser. No. 08/588,073, filed on Jan. 17, 1996, which issued onApr. 29, 1997 as U.S. Pat. No. 5,625,653, which is a continuation ofapplication Ser. No. 08/347,835, filed on Dec. 1, 1994, which issued onFeb. 27, 1996 as U.S. Pat. No. 5,495,508, which is a continuation ofapplication Ser. No. 08/104,322, filed on Aug. 9, 1993, now Abandoned,which is a continuation of application Ser. No. 07/438,618, filed onNov. 20, 1989, now Abandoned, which is a continuation of applicationSer. No. 07/123,395, filed on Nov. 20, 1987, which issued on Jun. 19,1990 as U.S. Pat. No. 4,935,927.

BACKGROUND

[0002] In general, present day telephone systems are increasingly usingwireless technology for long distance calls and, in some instances, havebegun the use of digital technology; however, no system in general usetoday has been capable of providing effective and efficient wirelessdigital technology for local calls to and from individual subscribers.Such technology has been disclosed in various recent patents commonlyowned by the present applicants' assignee, as, for example, in U.S. Pat.No. 4,644,561, dated Feb. 17, 1987 and U.S. Pat. No. 4,675,863, datedJun. 23, 1987. The technology disclosed in these patents provides basestations in communication with both a central office and a plurality ofsubscriber stations utilizing digital wireless time division circuitswherein there are repetitive sequential slot positions in a transmitchannel bit stream/ each slot being associated with a particularsubscriber.

[0003] The base stations used in the above time division system arerelatively complex and expensive but economically feasible for a largesystem serving a large number of subscribers; however, for relativelysmall systems serving a relatively small number of subscribers it may beeconomically infeasible. In addition, such a system utilizes a pair offrequencies, one for transmission and one for reception, and, in view ofthe limited amount of channels available in the spectrum, it would behighly advantageous if only one frequency could be effectively used.

[0004] It is, therefore, an object, of the present invention to providewhat may be called a simulated or emulated base station which can beeffectively substituted for an actual base station in certainsituations.

[0005] Another object is to provide a system that can be utilized forplural subscribers but which is operable on only a single frequency.

[0006] Other objects will become apparent from the following descriptionand claims.

SUMMARY

[0007] In essence, the system of the present invention utilizes what is,in effect, a modified subscriber station to act as a simulated oremulated base station, thereby considerably decreasing the total costand complexity of the system. This emulated base station essentiallydiffers from the subscriber station only in being able to initiate thesynchronization process, whereas the subscriber unit only acts to scanthe RF signals sent out by the emulated base station until it finds thefrequency and slot assigned to it. In the intervals betweentransmissions of the RF signals the emulated base station is adapted toreceive RF signals from the subscriber units. In this manner, thesubscriber unit may either talk to the emulated base station which thenacts as another subscriber station, or it may talk to another subscriberstation that has been synchronized therewith by the emulated basestation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a block diagram showing an overall system embodying thepresent invention.

[0009]FIG. 2 is a diagramatic illustration of the RCC waveform used inthe standard base station.

[0010]FIG. 3 is a diagramatic illustration of the RCC waveform used inthe present invention.

[0011]FIG. 4 is a diagramatic illustration showing the positive edges ofthe amplitude of the received signal used in course synchronization ofthe present invention.

[0012]FIG. 5 is a block diagram of the circuit for obtaining coursesynchronization in the present invention.

[0013]FIG. 6 is a block diagram of the received AGC circuit used in thepresent invention.

[0014]FIG. 7 is a block diagram showing the frequency acquisitioncircuit used in the present invention.

[0015]FIG. 8 is a diagramatic illustration of a wireless phone systemconfiguration embodying the present invention.

[0016]FIG. 9 is a diagramatic illustration similar to FIG. 8 but showinga dual subscriber system.

[0017]FiG. 10 is a diagramatic illustration of the frame format of thedual subscriber system of FIG. 9.

[0018]FIG. 11 is a diagramatic illustration of the frame format of aplurality of dual subscriber systems.

[0019]FIG. 12 is a diagramatic illustration of a system embodying thepresent invention which is used for monitoring one or more functions.

[0020]FIG. 13 is a diagramatic illustration of a repeater systemembodying the present invention.

[0021]FIG. 14 is a diagramatic illustration of a system embodying thepresent invention utilizing multiple repeaters.

[0022]FIG. 15 is a diagramatic illustration of a system embodying thepresent invention where a single repeater is used to drive a pluralityof other repeaters as well as subscriber units.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The overall internal operation of the system, generallydesignated 10, is shown in block diagram form in FIG. 1. In this system,during a telephone conversation, a person speaks into the telephone 12and the speech signal is sent to the local telephone interface unit 14.The signal is digitized by the codec 16 and the resultant digital datastream is then fed to the speech processor 18 which compresses thespeech data to a lower data rate. The compressed data is then fed to themodem 20 via line 22 and double-throw switch 24, the modem is acting toconvert the data stream to a spectrally efficient analog signal. Thisanalog signal is fed to the radio 26 via line 28. The radio upconvertsthe signal to a radio frequency (RF) signal and then transmits this RFsignal via the antenna 30.

[0024] In the intervals between transmissions of the RF signals, theunit is adapted to receive RF signals from a subscriber unit. The radio26 downconverts each of these RF signals to an IF signal and feeds thisIF signal to the modem 20 via line 32. The modem 20 demodulates the IFsignal to form a digital signal which is then fed to the speechprocessor via switch 24 and line 36. The speech processor thereupon actsto expand the signal to a digitized speech signal and this digitizedsignal is then fed into the codec 16 which outputs an analog speechsignal to the telephone 12 via the telephone interface 14.

[0025] The data transmission mode is similar to that described aboveexcept that the telephone is replaced by a data terminal or computer 38and the telephone, codec and speech processor are bypassed by means ofthe alternate position of the switch 24 that is then coupled to theterminal 38 by lines 40 and 42.

[0026] The modem 20 and radio 26 are both coupled to a control unit 44.The control unit 44 includes selecting means 45, monitoring means 46 andassigning means 47. The control unit 44 includes selecting means 45,monitoring means 46 and assigning means 47. The control unit 44 isinitially set to a predetermined slot, modulation and training mode forthe modem and to a predetermined RF frequency and power level for theradio. However, these parameters can be adjusted by the subscriber unitin the event they are not adequate to provide a satisfactory receptionat the subscriber station.

[0027] In a system utilizing an actual base station, such as, forexample, the system described in the aforesaid U.S. Pat. No. 4,675,863,the transmitted waveform is divided into a multiplicity (i.e. 45) msec.frames. Each frame is, in turn, divided into four 11.25 msec. slots. Thebase station transmits on all four slots to produce a 100% duty cyclemodulation waveform, the lone exception being the radio control channel(RCC). The RCC slot is slightly shorter than 11.25 msec and this causesa small gap in the modulation at the beginning of every frame. This gapis known as an AM hole. A diagram of the waveform of the RCC channel inthe actual base station format is shown in FIG. 2. In the system of thepresent invention, however, there is no transmission of a 100% dutycycle waveform. Instead, there is a transmission on only one slot perframe (a 25% duty cycle waveform), as shown in FIG. 3. This modifiedframe format necessitates changes in coarse synchronization, automaticgain control (AGC) and frequency acquisition. These changes areindicated in the following description:

[0028] Coarse Synchronization

[0029] Since the system of the present invention utilizes only a 25%duty cycle waveform, it monitors the amplitude of the received signaland searches for positive edges in the amplitude signal. These positiveedges are illustrated in FIG. 4. The subscriber unit adjusts its frametiming to align with the occurrence of these positive edges.

[0030] The circuit for obtaining the above type of coarsesynchronization is shown in block diagram form in FIG. 5 where thereceived signal is shown as being fed into an amplitude computationdevice 50 which produces a computer amplitude signal that is then passedto a comparator 52 where it is compared to a predetermined thresholdsignal, thereby forming a digital signal (1=signal present, 0=no signalpresent). This digital signal is fed into an edge detector 54 thatoutputs a strobe to indicate the detection of a positive edge.

[0031] AGC

[0032] The 25% duty cycle modulation requires a distinct type of receiveAGC circuit which avoids tracking when there is no signal present. Aslow rise fast decay AGC is, therefore, provided. This is shown in FIG.6 where the received signal is fed into an amplitude computation device56, which may take the form of a pre-programmed ROM, from which aresulting amplitude signal is fed into a comparator 58 in which it issubtracted from a predetermined threshold value to form a differencesignal. This difference signal is fed through one of two scalingmultipliers, shown at 60 and 62, into a low pass filter comprising anadder 64 and a delay means 66 connected through a loop 68. One or theother of the two multipliers is used in accordance with the sign of thedifference signal. If the difference signal is positive, the slow decayin the AGC control signal is implemented. If the difference signal isnegative, a fast rise in the AGC control signal is implemented. Theoutput of the filter is the gain signal which is then fed to the gaincontrol unit 44 shown in FIG. 1.

[0033] Coarse Frequency Acquisition

[0034] Since in the 25% duty cycle frame format it is not required toperform frequency acquisition during the off time (75% null time) andsince the frame timing is not known at the time when frequencyacquisition is performed, a modified form of frequency acquisitioncircuit has been provided, as shown in FIG. 7. In this circuit thereceived signal is fed into a Discrete Fourier Transform (DFT)computation device 70 which outputs the high band energy (energy in thefrequency band above the center frequency) and the low band energy(energy in the frequency band below the center frequency). The high bandenergy output is subtracted from the low band energy output at the adder72 and the output thereof is fed to a mixer or multiplier 74. Thereceived RF signal is also passed to a stripping means 76 which stripsoff the sign of the signal (negative or positive), thereby determiningonly the amplitude of the signal. The stripped signal is then fed to afilter 78 which smooths the signal by averaging it out. The output fromthe filter 78 is fed, via amplifier 80, to the multiplier 74.

[0035] The primary purpose of the circuit through 76, 78, and 80 is toprevent the action of noise on the output signal while accentuating thesignal itself. In this respect, since noise generally has a smallamplitude, it is effectively filtered out during the smoothing process.On the other hand since the actual signal generally has a relativelylarge amplitude it is, in effect, highlighted by adding the smoothed orfiltered signal to the mixer 74.

[0036] The scaled signal leaving the mixer 74 is balanced between thehigh and low energy frequencies, and this balanced signal, that-isproportional to the short term average amplitude of the received signal,is fed into a lowpass filter comprising an adder 82, and a delay means84 which are looped at 86. The delay means 84 causes the output signal88 to the VCXO control to represent the output immediately prior to theoutput actually fed into the lowpass filter. The VCXO control is used toadjust the frequency of the master oscillator in the system.

[0037] After initial or course synchronization has been effected, thesystem is in an idle voice mode but is fully set up for voice operation.If the phone at either end goes off-hook, the phone at the other endwill ring until the ringing phone is answered or the initiating phonegoes on-hook.

[0038] The calls are set up by a voice code word (VCW) at the beginningof every voice slot, this code word indicating an off-hook condition atthe initiating station. When this occurs, the station acting as anemulated base station then appears to itself go off-hook to the centraloffice (CO) thereby making a connection to the central office. Theinitiating subscriber station then proceeds to complete the call bydialing the desired number. When the initiating subscriber unit goeson-hook, the emulated base station is so informed by the VCW andpresents an on-hook appearance to the central office.

[0039] When the emulated base station detects a ring signal from thecentral office, the subscriber unit is caused to ring by means of thecorresponding VCW from the emulated base station. When the subscriberunit thereafter goes off-hook, the emulated base station is so informedvia the corresponding VCW and it then presents an off-hook appearance tothe central office.

[0040] The above type of wireless phone system configuration isexemplified in FIG. 8 where the subscriber unit 90 is shown in wirelesscommunication via antennas 92 and 94 with the emulated base station 96.The station 96 is in wireline communication via line 98 and interface100 with the central office.

[0041] Dual Subscriber System

[0042] The above-described system can be employed with a dual subscriberarrangement as shown in FIG. 9. In this system each channel is capableof supporting two complete conversations without the necessity of usinga duplexer. In this respect, a dual subscriber unit 102 is connected bywires 104 and 106 to a pair of subscriber telephone sets 108 and 110.The subscriber unit 102 is in wireless communication via antennas 112and 114 with an emulated dual base station 116. The unit 116 isconnected to the central office by wire lines 118 and 120.

[0043] The two separate subscribers 108 and 110 utilize a time slotarrangement, such as disclosed in the aforesaid U.S. Pat. No. 4,675,863,wherein each subscriber is assigned a separate slot. The frame formatfor this arrangement is shown in FIG. 10 where four slots are shown,numbered 1, 2, 3 and 4. The first two slots are used for the emulatedbase station and the last two are used for the two subscribers.

[0044] A plurality of dual subscriber systems may be operated ondifferent channels without duplexers by synchronizing all of theemulated base station transmissions. This is illustrated by the frameformat shown in FIG. 11 where channel 1 is shown above and channel n(indicating any desired number of channels in between) is shown below.On each channel, the first two slots are for transmission and the lasttwo are for reception.

[0045] Paged Remote Service

[0046] One emulated base station may be used with a plurality ofdifferent subscribers, one at a time. In such arrangement, forreception, the subscribers continuously monitor the transmissions of theradio control channels (RCC), described more fully in the aforesaid U.S.Pat. No. 4,675,863, until a particular subscriber is paged by theemulated base station by means of the subscriber's ID Number (SID).After receiving a page, the subscriber initiates a transmission back tothe emulated base station using the synchronization process describedabove. For initiating a call, the subscriber transmits on the RCC usingthe previously described synchronization process.

[0047] Monitoring Function

[0048] The present system may be used for monitoring one or morefunctions. In this respect, using a computer as a controlling/datalogging device, a plurality of subscribers may be periodically polled toreport on some function such as temperature, weather conditions,security, water/flood warnings, low fuel warnings, remote gas, electricor water meter readings, etc. This is illustrated in FIG. 12 where anemulated base station 122 is in wireless communication with a pluralityof subscriber units respectively designated 124, 126 and 128. The unit122 is in wire line connection with both a telephone 130 for voicecommunication and a computer or data terminal 132 for data input.Similarly, each subscriber unit is connected both to a respectivetelephone 134, 136 or 138 for voice communication and to a data device,as at 140, 142 or 144 respectively.

[0049] Repeater System

[0050] An important use of the present system is as a repeater to extendthe range of the system. In this arrangement, the emulated base stationmay be used to overcome interfering obstacles such as mountains and thelike. FIG. 13 illustrates this function, showing a subscriber unit 146in wireless communication with an emulated base station 148 on thesummit of a mountain. The unit 148 is also in wireless communicationwith a standard base station 150 connected to a central office.

[0051] The relative simplicity and inexpensiveness of the emulated basestation makes it very cost effective as a repeater unit. It can also beused as a repeater to extend the long distance range of the systemregardless of the presence or absence of obstructions. By utilization ofthe time slot arrangement, the repeater unit, without -the use of anyduplexer, fits into the complete system while remaining transparent toboth the standard base station and the subscriber. It can, of course,also be interposed between the subscriber and another emulated basestation instead of a standard base station. This can be provided inmultiple stages from one emulated base station to another to greatlyincrease the range of the system in a relatively inexpensive manner.This is illustrated in FIG. 14 where a series of repeater units 152 areinterposed between the subscriber 154 and the base station 156.

[0052] In addition to extending the range of the system, the repeaterunit serves to clean up the actual base station signal via equalizationbefore retransmission to the subscriber.

[0053] One repeater can also be used in what may be termed a repeaterstar system to drive multiple repeaters and/or subscribers. This isillustrated in FIG. 15 where the single repeater unit 158 is in wirelesscommunication with ancillary repeaters 160 and 162 as well as with oneor more subscribers such as at 164. The ancillary repeaters arethemselves in wireless communication with subscribers such as shown at166, 168, 170, 172 and 174 as well as with other ancillary repeaterssuch as at 176. Any one of the ancillary repeaters, such as repeater162, may be used as the final repeater in direct communication with thebase station indicated at 178.

[0054] Multiple repeaters may be placed at one location, oil differentchannels and synchronized so that their transmissions and receptionsoccur simultaneously, thereby avoiding the use of duplexers. In such aconfiguration, a master repeater is used to monitor the RCC channel ofthe base station and relays the monitored information to the varioussubscribers via the emulated base station's RCC. In such aconfiguration, on call setup, the subscribers are each assigned arepeater channel.

What is claimed is:
 1. A method for conducting a plurality of wirelesscommunications which comprise transmitted (TX) and received (RX)information signals using time division multiple access wirelesstransmission and reception, the method comprising: transmittingsynchronization information and the assignment of periodic slots from afirst communication station to a second communication station; receivingsaid transmitted synchronization information and said assignment ofperiodic slots at said second communication station; adjusting thetiming of said second communication station, whereby the timing of saidfirst communication station and said second communication station aresynchronized; transmitting synchronization information and theassignment of periodic slots from said second communication station to athird communication station; and adjusting the timing of said secondcommunication station, whereby the timing of said first communicationstation, said second communication station, and said third communicationstation are synchronized.
 2. The method of claim 1 , further comprisingthe steps of: transmitting information signals from said firstcommunication station to said second communication station in a firsttime period, and receiving information signals at said firstcommunication station from said second communication station in a secondtime period, whereby said first communication station and said secondcommunication station alternately transmit and receive in a synchronizedtime division duplex scheme.
 3. The method of claim 2 , wherein saidinformation signals comprise voice signals.
 4. The method of claim 3further comprising the steps of: receiving an analog voice signal atsaid first communication station; digitizing said analog voice signal atsaid first communication station, thereby producing a digitized voicesignal; and compressing said digitized voice signal, thereby producing areduced bit rate signal.
 5. The method of claim 2 , wherein said firstcommunication station is in wireline communication with a centraloffice.
 6. The method of claim 1 , wherein with said secondcommunication station wherein said third communication station islocated beyond the transmission range of said first communicationstation.
 7. A method for conducting a plurality of telephoniccommunications which comprise transmitted (TX) and received (RX)information signals using time division wireless transmission andreception, the method comprising: listening at second communicationstation for a device access code; paging said second communicationstation from a first communication station by transmitting said deviceaccess code; receiving said page at said second communication station;responding to said page by transmitting from said second communicationstation to said first communication station; synchronizing the timing ofsaid second communication station to the timing of said firstcommunication station; and initiating a connection between said firstcommunication station and said second communication station, where saidfirst communication station and said second communication stationalternately transmit and receive in a synchronized time division duplexscheme.
 8. The method of claim 7 , further comprising the steps of:listening at a third communication station for a device access code;paging said third communication station from said second communicationstation by transmitting a device access code; receiving said page atsaid third communication station; responding to said page bytransmitting from said third communication station to said secondcommunication station; and synchronizing the timing of said thirdcommunication station to the timing of said second communicationstation.
 9. The method of claim 8 , further comprising the step ofinitiating a connection between said second communication station andsaid third communication station, wherein said second communicationstation and said third communication station alternately transmit andreceive in a synchronized time division duplex scheme.
 10. The method ofclaim 9 , wherein said third communication station is located beyond thetransmission range of said first communication station.
 11. The methodof claim 10 , wherein said second communication station transmitsinformation received from said first communication station to said thirdcommunication station and said second communication station transmitsinformation received from said third communication station to said firstcommunication station.
 12. The method of claim 7 , wherein saidinformation signals include voice signals or a data signals.